1. Field of the Invention
This invention concerns a thin film electroluminescent display device, in which an electroluminescent (hereinafter simply referred to as EL) emission layer is disposed between a transparent electrode and an opposing electrode and a voltage is applied to the EL emission layer to cause light emission.
2. Description of the Prior Art
Thin film El display devices have been applied to various kinds of displays in recent years and they are generally classified into AC driving and DC driving types. Thin film EL display devices in the prior art have generally been constituted, for example, as shown in FIG. 4 as a 8-layered structure having double insulation films, in which a transparent electrode 2, an insulation layer 3, an EL emission layer 4, an insulation layer 5 and an opposing electrode 6 are laminated successively on a glass substrate 1. The thin film EL display device is adapted such that an alternating electric field from several tens Hz to several KHz is applied between the transparent electrode 2 and the opposing electrode 6 to excite ions of activated species in the EL emission layer 4 to cause light emission.
The important feature of the thin film EL display device when it is used as a display, like other display devices (for example, liquid crystals), is that the displayed contents can clearly be observed visually under the brightness of sun light at the outdoor and under the brightness of various kinds of illuminations in a room. A metal film such as aluminum is used as the opposing electrode 6 of the thin film EL display device. Particularly, since the aluminum film has a high metallic gloss, an external light is reflected at the surface of the opposing electrode 6 in the thin film EL display device and observed together with the light emitted from the EL emission layer 4. Accordingly, in the case where the external light is highly bright, it is difficult to discriminate the emission portion from the non-emission portion of the EL emission layer 4 in the conventionally thin film EL display device, which makes it difficult to read the display.
In view of the above, various attempts have been made for improving the display contrast. For instance, it has been known to introduce black materials in the glass substrate 1 or attach a black filter. However, in such an EL display device, although the reflection light can surely be decreased since the side of the display surface is blackened, the EL emission is also decreased thereby, to provide only an insufficient effect for the improvement of the contrast.
Further, as shown in FIG. 5, there has also been known a device in which a black light absorbing layer 7 is disposed between the insulation layer 5 at the back of the EL emission layer 4 and the opposing electrode 6. In this case, since the light absorbing layer 7 is disposed at the back of the EL emission layer 4, reduction in the EL light emission caused by the light absorbing layer 7 can be alleviated. However, as shown in FIG. 6, the incident light such as illumination light is tended to be partially reflected at the interface between the insulation layer 5 and the light absorbing layer 7 and the reflection light at the interface between the insulation layer 5 and the light absorbing layer 7 is observed as a black metallic color on the side of the display surface, which results in no sufficient contrast.